1. Field of the Invention
The invention is generally related to drilling and producing oil offshore and more particularly to floating structures used in such operations.
2. General Background
In the offshore oil industry, floating structures are used in areas where deep water results in the cost of a jacket fixed to the sea floor being too expensive to realize a sufficient economic return, even for large oil reserves. Such floating structures have been semi-submersibles, a column stabilized vessel that is moored in place by the use of multiple anchors, single column spar type structures, that are moored in place by multiple anchors, dynamically positioned vessels that use a number of thrusters to hold the vessel in position at the site, and tension leg platforms (TLP's).
Each structure has advantages and disadvantages. For example, while dynamically positioned vessels eliminate the need for anchors and mooring lines, they present a large surface area to waves and currents, which can result in a substantial amount of power required to hold the vessel in position. The large surface area also results in the vessel being subject to heave, pitch, and roll motions in response to wave action. The semi-submersibles present less surface area to waves and so are less susceptible to pitch and roll motions but are still subject to heave motions and are not designed to store large quantities of oil.
Minimizing environmentally induced motions is desirable not only from a safety and comfort standpoint, but also from an operational standpoint since drilling and producing through risers which are connected from the vessel to the sea floor wellhead must be designed to accommodate the motions of the structure. The cost of designing and building risers is directly related to the amount of heave, pitch, and roll of the structure, as well as the wave, current, and gravity forces acting on the risers themselves.
The TLP is relatively successful at minimizing heave, pitch, and roll. However, the TLP is a relatively shallow draft structure that is expensive and limited to moderate water depths. Further, it is virtually immobile once it has been installed.
The spar type structures (a single column hull such as that described in U.S. Pat. No. 4,702,321) are subject to vortex induced vibrations in high currents. This has been dealt with by including helical strakes along the length of the hull. Due to the large diameter of the hull, these structures must be built at a specially equipped construction facility. Also, as the diameter of these structures becomes larger, fabrication becomes more difficult. Transportation of a large spar type structure to the installation site, whether on a heavy lift vessel or by floating the completed hull, may also present difficulties. The disadvantages of the strakes required on single column spar structures in high currents are that they increase cost and increase drag, which in turn increases the cost of mooring.